Should this work with tubeless tires? Seems to be leaking from the valve stem…

No you will need the liquid sealant to plug the pores in the tyre (when they are a non UST tyre) and the valve.

I can inflate them easily but then they go down after a little while, with tubeless or non-tubeless tyres.

My 355s with yellow tape didn't seal until I put sealant in – they leaked around the valve. Been a few months now, still perfect.

Why would you want to run tubeless without sealant anyway?

Yeah I don't get that either

It's the thought of mucking about with sealant that's stopped me going tubeless up to now.

I hate sludge, slime, snot etc.

I find the sealant to be annoying. I do think it's easy enough to set up, but for some reason I change tires a lot, and I feel that has quite a lot of extra work. There are no thorns here requiring sealant as a back up, I'd rather just carry a tube in my back pack (I guess I'd do that even if I had sealant).

I also thought it was strange that it leaked from the valve stem, you would imagine that would be easy to get right? I recollect my Shimano XT wheel set having a seal on the inside and outside, this one only had one on the inside.

Would a rimstrip added to the yellow tape make a difference? Tires are LUST highrollers if it makes a difference.